CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair of T91 Steel for Ultra-Supercritical Turbine Rotors
Material Background and Repair Challenges
T91 steel (9Cr-1Mo-V-Nb, UNS K91001) is a martensitic heat-resistant alloy widely used in ultra-supercritical (USC) and supercritical steam turbine rotors operating at temperatures up to 650°C and pressures exceeding 25 MPa. The excellent high-temperature creep strength and oxidation resistance of T91 stem from its fine, stable carbide dispersion (MC and M23C6) and controlled microstructure. However, turbine rotors are subject to thermal fatigue, creep damage, and localized material loss due to corrosion or erosion, necessitating periodic repair and cladding.
The repair of T91 steel presents unique challenges that distinguish it from conventional carbon steel or austenitic stainless steel welding:
- High hardenability: The 9% Cr content and alloying elements promote the formation of hard, brittle martensite in the heat-affected zone (HAZ), leading to cracking susceptibility.
- Residual stress sensitivity: T91 is prone to hydrogen-assisted cracking and temper embrittlement if not properly heat treated.
- Microstructural evolution: The HAZ and weld metal must maintain the required creep strength and ductility balance after repair welding.
- Dilution control: When cladding with a different alloy (e.g., austenitic stainless steel for corrosion resistance), dilution from the base metal can compromise the cladding layer's properties.
Welding Process Selection and Parameter Control
The selection of welding process for T91 repair is critical. Submerged arc welding (SAW) is commonly used for thick-section repairs due to its high deposition rate and deep penetration, while gas tungsten arc welding (GTAW) is preferred for root passes and thin-section repairs due to its precise heat input control. Gas metal arc welding (GMAW) with solid or flux-cored wire is increasingly used for its productivity and flexibility.
| Welding Process | Typical Application | Heat Input Range | Filler Material |
|---|---|---|---|
| GTAW | Root pass, thin sections | 0.5–1.5 kJ/mm | ER911, ER911S |
| SAW | Bulk fill, thick sections | 2.0–5.0 kJ/mm | S911, S911L |
| GMAW | Multi-pass fill | 1.0–3.0 kJ/mm | ER911, ER911S |
| PTA Cladding | Corrosion overlay | 1.0–2.5 kJ/mm | NiCrBSi, Inconel 625 |
Preheating is a mandatory step for T91 repair welding, with typical preheat temperatures ranging from 200°C to 300°C depending on section thickness and restraint level. Interpass temperature must be maintained within a similar range to prevent excessive cooling rates that promote brittle martensite formation. Post-weld heat treatment (PWHT) is essential, typically performed at 720–760°C for 2–4 hours per 25 mm of thickness, followed by controlled cooling.
Microstructural Considerations and Defect Prevention
The microstructure of the weld and HAZ in T91 repair is the primary determinant of long-term service performance. The ideal microstructure consists of tempered martensite with fine, evenly distributed carbides. Deviations from this microstructure—such as retained austenite, coarse carbide precipitation, or delta ferrite—can significantly degrade mechanical properties.
Common defects and their countermeasures include:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| HAZ cracking | High cooling rate, excessive restraint | Increase preheat, reduce heat input, optimize PWHT |
| Weld metal cracking | Hydrogen pickup, poor ductility | Low-hydrogen consumables, proper preheat |
| Lack of fusion | Insufficient heat input, poor technique | Increase current, optimize travel speed |
| Excessive hardness | Rapid cooling, insufficient tempering | Ensure proper PWHT, control interpass temperature |
| Creep cavity formation | Long-term exposure at high temperature | Optimize microalloy content, control grain size |
The repair of turbine rotors also involves the challenge of geometric restoration. Rotors are precision components with tight dimensional tolerances, and the repair must not compromise the rotational balance or stress concentration factors at key locations such as fillets and keyways.
Engineering Practice and Inspection
In practice, T91 rotor repair welding requires strict qualification and documentation in accordance with ASME Section IX or equivalent national standards. Welder qualification tests must include mechanical property testing (tensile, impact, hardness) and non-destructive examination (RT or UT) of the qualification coupon. For in-service repairs, the inspection regime includes:
- Pre-repair: Detailed examination of the damage extent using magnetic particle testing (MT) or ultrasonic testing (UT).
- Post-weld: 100% UT of the repair area, hardness survey across the weld and HAZ.
- Post-PWHT: Final dimensional check, hardness re-verification, and mechanical property testing if required.
The long-term performance of T91 repairs is validated through surveillance testing and periodic in-service inspections, including dimensional measurement of critical areas and hardness trending.
Summary
The cladding repair of T91 steel turbine rotors is a high-consequence operation that demands rigorous process control, thorough material understanding, and meticulous quality assurance. The key success factors are proper preheat and interpass temperature control, selection of appropriate filler materials, adequate PWHT, and comprehensive inspection. Engineers must recognize that the repair does not simply restore geometry but must also restore the material's mechanical properties and long-term creep resistance to meet the demanding service conditions of ultra-supercritical power plants.